What's the use of private constructor in Java?

Question

I want to know that how private constructor is useful in Java. What are the different ways to use private constructor in Java?

Answer 1

private constructor is off course to restrict instantiation of the class.

Actually a good use of private constructor is in Singleton Pattern. here's an example

public class ClassicSingleton {    private static ClassicSingleton instance = null;    private ClassicSingleton() {       // Exists only to defeat instantiation.    }    public static ClassicSingleton getInstance() {       if(instance == null) {          instance = new ClassicSingleton();       }       return instance;    } } 

this way you can ensure that only one instance of class is active.

Other uses can be to create a utility class, that only contains static methods.

For, more analysis you can look into other Stack overflow answers

Can a constructor in Java be private?

What is the use of making constructor private in a class?

private constructor

Answer 2

As other answers mentioned, common uses include the singleton pattern, internal constructor chaining and one more:

Java doesn't support what in C# (for example) is known as a "static class" - in other words, a utility class. A utility class is a helper class that's supposed to contain only static members. (Math and System are such cases in Java.) It doesn't make sense for them to be instantiated in any way.

In C#, making a class static makes it implicitly both final/sealed and abstract. In Java, there is no such keyword and you can't make a class final and abstract. So if you had such a utility class, you'd make it final and give it a private constructor that's never called.